As typical bacteria, which exhibit acid resistance in combination with heat resistance, there are known Alicyclobacillus acidoterrestris, Alicyclobacillus acidocaldarius, etc. The spores of these bacteria possess resistance against the normally employed pasteurization method for fruit juice. Therefore, in recent years, the spoilage of fruit juice caused by Alicyclobacillus acidoterrestris has been a serious problem throughout the world.
In order to suppress growth of the bacterium in fruit juice, it is effective to add a synthetic preservative such as benzoic acid. However, there is now strongly demanded a material with an enhanced degree of safety, which is derived from a natural product but not from a synthetic product.
As an antimicrobial agent, which is derived from a natural product and is effective against the bacterium, there has been reported only nisin, which is a peptide derived from lactic acid bacterium (International Journal of Food Science and Technology, vol. 34, pp. 81 to 85, 1999). However, nisin contains a special type of amino acids such as dehydroalanine.
Consequently, for the preservation and storage of fruit juice, there is demanded an antimicrobial agent, which is derived from a natural product and is effective at lower concentration, against acid-resistant and heat-resistant bacteria such as Alicyclobacillus acidoterrestris.
The inventor of the present invention has studied characteristics of alpha-type thionin and beta-type thionin, which are peptides derived from wheat and barley. To the best of the inventor's knowledge, no one has reported any antimicrobial activity of thionins against Alicyclobacillus acidoterrestris, which is an acid-resistant and heat-resistant bacteria, while its antimicrobial activity against phytopathogenic fungi has previously been known (Plant Science, vol. 92, pp. 169 to 177, 1993). In addition, such thionins do not contain any special amino acid at all, unlike the above-mentioned nisin derived from lactic acid bacteria.